Abstract:

This study describes the composition of forest landscapes surrounding
northern spotted owl (Strix occidentalis caurina) nests in the central Cascade
Mountains of Oregon. I compared forest composition around 126 owl nests in 70 pair
territories with forest composition around 119 points drawn randomly from all
terrestrial cover-types, and around 104 points drawn randomly from the old-forest
(closed canopy, > 80 yrs) cover type. All nest sites and random points were drawn
from U.S. Forest Service lands and were not drawn from privately owned lands or
Wilderness Areas.
Forest cover was classified on a Landsat Thematic Mapper image. I quantified
the percentage of old-forest within 200 concentric circular plots (0.04-5.0-km radii),
centered on each analyzed point, using a geographic information system. I used
logistic regression to make spatially-explicit inferences.
Owl nests were surrounded by more old-forest when compared to points
drawn randomly from all terrestrial cover types: there was significantly (P<0.05)
more old-forest around the owl nests in plots as large as 1.79 km in radius. When
compared to points drawn randomly from the old-forest cover type, owl nests were
surrounded by significantly (P<0.05) more old-forest in plots with 0.17-0.80-km
radii.
Exploratory analyses suggest that the landscape scales most pertinent to
northern spotted owl nest site positioning in this study area appear to be (in
descending order): the surrounding 10-15 ha (~200-m radius), the surrounding 25-30 ha
(~300-m radius), the surrounding 200 ha (800-m radius), and possibly the surrounding
700 ha (1,500-m radius).
This study supports the assertion that northern spotted owls are strongly
associated with older forests. The results also indicate that owl nests are most
associated with higher proportions of old-forest near the nest implying that the
arrangement of habitat is important for nest-site selection/positioning Since spotted
owls in the central Cascade Mountains of Oregon are known to have home-ranges that
average 1,769 ha, it is important to recognize that these results apply to nest-site
selection/positioning on the landscape and not to the amount of habitat necessary for
pair persistence or successful reproduction.